1. Field of the Invention
The present invention relates to a microswitch and to a method of fabricating the microswitch, and in particular, to a microswitch for turning ON and OFF signals ranging from DC to AC current having a broad range of signal frequencies up to several hundred GHz, and to a method of fabricating such a microswitch.
2. Description of the Related Art
The prior art will first be explained taking as an example the invention disclosed in Micro Electromechanical RF Switch (Japanese Patent Laid-open No. 17300/97, U.S. Pat. No. 5,578,976, by Jun J. Yao of Rockwell International Corporation.
A plan view of the microswitch of the invention is shown in FIG. 1, and a cross section (taken along line C-Cxe2x80x2) is shown in FIG. 2. In this microswitch, anchor structure 144 composed of thermo-setting polyimide, lower electrode 146 and signal lines 148 both composed of gold are provided on gallium arsenide (GaAs) substrate 149. Cantilever arm 140 composed of a silicon oxide film provided on anchor structure 144 extends as far as the position of lower electrode 146 and signal lines 148 and confronts these components with an interposed spatial gap. Upper electrode 141 composed of aluminum is provided on the upper surface of cantilever arm 140 from a position opposite anchor structure 144 to a position confronting lower electrode 146. In addition, contact 142 composed of gold is provided on the lower surface of cantilever arm 140 at a position confronting signal lines 148.
When a voltage of 30 V is applied between upper electrode 141 and lower electrode 146, an electrostatic force works to attract upper electrode 141 toward the substrate, whereby cantilever arm 140 bends downward and contact 142 contacts signal lines 148. As shown in FIG. 1, a gap is provided in signal lines 148 at a position confronting contact 142.
Current does not flow in signal lines 148 in the state in which voltage is not applied between upper electrode 141 and lower electrode 146, but current can flow in signal lines 148 in the state in which voltage is applied between upper electrode 141 and lower electrode 146 and contact 142 contacts signal lines 148. In this way, ON/OFF control of the flow of a current or signal through signal lines 148 can be effected by the application of voltage.
In this case, sufficient electrical isolation between upper electrode 141 and contact 142 is critical for reducing loss in the switch. In other words, the problem exists that a portion of the signal (including DC) that flows through signal lines 148 flows to upper electrode 141 in the event of an electrical short-circuit between upper electrode 141 and contact 142. Even without a short-circuit between upper electrode 141 and contact 142, a considerably large electrostatic capacity between these two components inevitably results in the flow of a portion of the AC signal that flows through signal line 148 to upper electrode 141 and to the outside. When these two components are not adequately isolated, signal leakage increases and the characteristics of the switch deteriorate.
Typically, an electrostatic switch is required to exhibit high impedance when the switch is OFF and further, to allow switching between ON and OFF by the application of a low voltage. A switch that implements switching of an RF signal therefore necessitates an increase of the distance between signal line 148 and contact 142 of the switch that is provided above signal line 148 to increase impedance when OFF. In the above-described prior art, the distance between contact 142 and signal line 148 is less than the distance between upper electrode 141 and lower electrode 146. Such a construction entails the problem that, when the distance between signal line 148 and contact 142 of the switch that is positioned above signal line 148 is increased, the distance between the voltage application components (upper electrode 141 and lower electrode 146) must also be increased, thereby necessitating the application of greater voltage to drive the voltage application components. Since electrostatic force decreases in inverse proportion to the square of the gap, reducing the distance between these voltage application components is crucial for reducing the drive voltage of the switch.
In the above-described example of the prior art, moreover, cantilever arm 140 is shaped so as to extend parallel to substrate 149. In this case, when electrostatic attraction works between lower electrode 146 and upper electrode 141 that is provided in the central portion of cantilever arm 140 to bend cantilever arm 140 toward substrate 149, the distance that contact 142 moves in the direction of substrate 149 is greater than the amount of deflection of upper electrode 141 at the position that confronts lower electrode 146, this distance being the product of the amount of deflection of upper electrode 141 and a spring ratio (the distance between contact 142 and the base of anchor structure 144 of cantilever arm 140 divided by the distance between lower electrode 146 and the base of anchor structure 144 of cantilever arm 140). Thus, when the switch is turned ON, contact 142 first contacts signal line 148 on the right side of FIG. 2. If cantilever arm 140 is constructed with sufficient flexibility, all of contact 142 can be caused to contact signal line 148. However, it has been found that generally, the electrostatic force that drives the switch is small while the rigidity of cantilever arm 140 is rather great. As a result, the problem was encountered in the prior-art construction that contact between contact 142 and signal lines 148 was insufficient, impedance was not sufficiently low when the switch was turned ON, and signal loss was therefore great. Furthermore, in the construction of above-described example of the prior art, there also occurred the problem of one-sided contact in which contact 142 made contact with only one of signal lines 148 and did not contact the other signal line 148 when the switch was turned ON. It was found that this problem is related to the fact that the dimensions of narrow portion 143 of the connecting portion between cantilever arm 140 and anchor structure 144 are determined by the voltage that is applied between upper electrode 141 and lower electrode 146 and the lack of freedom in the design of contact 142. This point will be explained in greater detail in the embodiments of the present invention.
In addition to these problems, it was found that the prior-art example has the following problems that arise from materials and fabrication processes. Cantilever arm 140 (made from silicon dioxide) of the prior-art example contacts the different materials of upper electrode 141 (made from aluminum) and anchor structure 144 (made from polyimide) over an extensive area. Because this cantilever arm 140 is designed as a mechanically flexible construction in order to decrease the drive voltage, the slight strain that arises between these different materials tends to cause a high degree of warping. The strain that causes warping largely depends on the differences in thermal expansion coefficients of the different materials and on differences in processing conditions. The silicon dioxide in the prior-art example has a thermal expansion coefficient that differs by approximately 100 times from those of aluminum and polyimide. Warpage tends to occur easily due to processing temperatures as well as to temperature changes in the ambient atmosphere after completion of the device. Fabrication conditions such as the film thicknesses of the cantilever arm and anchor structure therefore must be accurately controlled to control warpage during fabrication, and these requirements result in increased fabrication costs.
Furthermore, since the completed device is subject to the influence of temperature changes in the atmosphere, problems have arisen relating to long-term reliability such as the failure of the switch when the drive voltage fluctuates or upon the occasional application of the maximum drive power supply.
The cantilever arm structure may be modified without varying the spring rigidity by increasing the thickness of the arm if the arm width is reduced. The overall switch dimension can thus be reduced by decreasing the arm width, thereby obtaining the advantage of enabling a larger number of switches in a small area. In the prior-art example in which silicon dioxide was used in the cantilever arm, however, there exists a severe restriction on increase of the thickness of the cantilever arm. In principle, the thickness of a silicon dioxide film can be increased to 10 xcexcm or more by increasing the growth time in a plasma enhanced chemical vapor deposition system (PECVD), but increasing growth time decreases the device processing speed and increases costs. In addition, extraneous matter tends to occur inside the device, and this leads to further problems regarding maintenance such as the need for frequent cleaning. Still further, the occurrence of greater internal strain inside a thick film gives rise to the problem of damage to the substrate during deposition. For these reasons, current practical considerations limit the thickness of the silicon dioxide film to just 2 xcexcm. As a result, the only method of making the switch structure more compact in the construction of the prior-art example was to shorten the length of the arm. Miniaturization of the device becomes problematic when this arm length is restricted by other demands on the device, and the design of the device is therefore subject to severe limitations.
The present invention was devised to solve these problems and has as its object the provision of a microswitch that features both high impedance when the switch is OFF and a low-voltage drive, and the provision of a method of fabricating the microswitch.
To achieve these objects, the microswitch according to the present invention is provided with: a first signal line provided on a substrate; a second signal line provided on the substrate and provided with an end that is separated from the end of the first signal line by a prescribed gap; a support that is fixed to the substrate;
a flexible arm that extends from the support; an upper electrode that is connected to the support by way of this arm; a lower electrode provided on the substrate in confrontation with the upper electrode; a dielectric structure that extends from the upper electrode; and a contact electrode provided on the dielectric structure in confrontation with the gap; the arm bending in accordance with voltage that is applied between the upper electrode and lower electrode and the microswitch thereby controlling conduction/nonconduction between the first and second signal lines; wherein the upper electrode, lower electrode, contact electrode, and signal lines are arranged such that the minimum distance between the contact electrode and signal lines is greater than the minimum distance between the upper electrode and lower electrode when the microswitch is in the OFF state.
Other modes of the microswitch according to the present invention include the constructions described hereinbelow. Specifically, the arm may extend from the support in a direction that is parallel to the substrate, and the dielectric structure may curve in a direction away from the substrate with increasing distance from the arm. Alternatively, the minimum distance between the lower surface of the contact electrode and the upper surface of the signal lines may be less than the minimum distance between the lower surface of the dielectric structure that is provided below the upper electrode and the lower electrode. Alternatively, the arm may curve away from the substrate with increasing distance from the support, and the dielectric structure may extend in a straight line from the arm. Alternatively, the arm may curve away from the substrate with increasing distance from the support, while the dielectric structure may have a shape that approaches the substrate with increasing distance from the arm.
A reinforcing plate may be provided on the dielectric structure at a position that confronts the contact electrode. Alternatively, a reinforcement structure may be provided on the dielectric structure between the contact electrode and the upper electrode. Alternatively, a reinforcing plate may be provided on the dielectric structure at a position that confronts the contact electrode, a reinforcement structure may be provided on the dielectric structure between the contact electrode and the upper electrode, and the reinforcement structure and the reinforcing plate may be connected. Alternatively, the dielectric structure that is provided between the contact electrode and the upper electrode may have a width dimension in the direction parallel to the direction in which the first and second signal lines extend that is less than the width of the contact electrode. Alternatively, at least one second upper electrode may be provided on the dielectric structure at a position opposite to the upper electrode with the contact electrode interposed between, and at least one second lower electrode may be provided on the substrate in confrontation with this second upper electrode. Alternatively, the dielectric structure may be directly connected to the substrate, and the thickness of the dielectric structure is uniform. Alternatively, the substrate may be made from a glass substrate.
The method of fabricating a microswitch according to the present invention is a method of fabricating a microswitch provided with: a first signal line provided on a substrate; a second signal line provided on the substrate and provided with an end that is separated from the end of the first signal line by a prescribed gap; a support that is fixed to the substrate; a flexible arm that extends from the support; an upper electrode that is connected by way of this arm; a lower electrode provided on the substrate in confrontation with the upper electrode; a dielectric structure that extends from the upper electrode; and a contact electrode provided on the dielectric structure in confrontation with the gap; the arm bending in accordance with voltage that is applied between the upper electrode and lower electrode and the microswitch thereby controlling conduction/nonconduction between the first and second signal lines; the fabrication method including steps of: forming the first and second signal lines along with the lower electrode on the substrate; forming a member composed of the support, the arm, the upper electrode, the dielectric structure, and the contact electrode; and attaching this member onto the substrate such that the contact electrode and the gap are in confrontation; wherein the upper electrode, lower electrode, contact electrode, and signal lines are arranged such that the minimum distance between the contact electrode and signal lines is greater than the minimum distance between the upper electrode and lower electrode when the microswitch is in the OFF state.
A method of fabricating a microswitch according to the present invention includes other modes corresponding to the above-described microswitches.
As described hereinabove, by providing a curve to the cantilever structure, the present invention increases the distance between the contact electrode and signal lines in the signal portion while keeping small the distance between the upper electrode and lower electrode in the drive portion. Adopting this configuration enables the simultaneous realization of the two requirements for high impedance when the switch is OFF and low-voltage drive that were contradictory in the prior-art example. Furthermore, electrical coupling between the upper electrode and contact electrode can be suppressed to a low level by separating the upper electrode and contact electrode by a sufficient distance using a dielectric film while mechanically connecting the two components. As a result, the switch of the present invention can be used in an RF circuit having a high frequency in the millimeter wave region.
In an embodiment of the present invention, the cantilever structure is divided into two portions, one being an arm that is connected to the support and the other being an acting portion that is connected to the tip of the arm, each portion being constituted by different materials. When the arm is made from a semiconductor material, for example, the use of a single material can solve the problem encountered in the prior art that temperature changes tend to cause changes in curvature of the arm. In addition, the freedom to adopt high-temperature processing conditions (greater freedom in processing) enables easy control of the thickness of the semiconductor material over a wide range and provides a solution to the problem of the prior-art example regarding the difficulty of fabricating a structure having a thickness greater than 2 xcexcm.
In another embodiment of the present invention, the provision of a reinforcement structure in an area of the dielectric film of the acting portion of the above-described cantilever structure other than the portion in which the upper electrode and the contact electrode are provided enables greater control of the shape of the cantilever structure. The provision of this reinforcement structure greatly increases the degree of design freedom compared to the shape control of the prior art that was realized only by the shape inherent to the dielectric film.
In another embodiment of the present invention, moreover, a drive portion is provided on both sides of the contact electrode. The attraction of each of the upper electrodes toward respective lower electrodes of these divided drive portions enables the realization of reliable contact between the contact electrode and the signal lines, thereby enabling reliable switch operation despite some variation in the curvature of the cantilever arm and allowing a solution to the problem of long-term reliability described with regard to the prior art.
In another embodiment of the present invention, a narrow portion is provided in the dielectric film that connects the upper electrode of the drive portion to the contact electrode of the contact point. Decreasing the width of this narrow portion facilitates the rotating motion of the contact electrode with the axis of the narrow portion as center and acts to prevent the problem of one-sided contact of the prior-art example. This narrow portion is at a different position than the arm that connects the cantilever structure to the support, and the narrow portion can therefore be designed independently of the voltage of the drive portion.
The above and other objects, features, and advantages of the present invention will become apparent from the following description based on the accompanying drawings which illustrate examples of preferred embodiments of the present invention.